Under normal circumstances, there should be no overflow. The
mechanical fuel pump is pressure limited (kind of like a pressure
regulator built-in.) The way it works is the eccentric on the cam
"cocks" the pump by pulling the diaphragm down and drawing fuel in.
Then the spring pushes the diaphragm back up, supplying a constant
pressure to the carbs. (Most electric tick-tick pumps operate this
way too, BTW. The rotary pumps are positive displacement, I think.)
If the overflow is dumping fuel, either the spring in the fuel pump is
too strong or the needle valve is leaking.
The vent tubes should be open to atmosphere, and only vapour should be
coming out there. Else the extra pressure will push liquid fuel
out through the bottom of the float bowl in to the jet and into the
carburettor throat.
HTH,
Donald.
> From: "George P." <ygpz4re@hotmail.com>
> Date: Thu, 30 May 2002 10:36:51 -0400
>
> Hmmmm.... You've actually said something here that has me wondering. You
> said, "they should just be there to balance the air pressure."
>
> My rear carb has a fuel inlet, fuel outlet, and vent tube (for now, I won't
> call it an "outlet" even though that's how I've been "using" it). The front
> carb has a fuel inlet and a vent tube. I've connected these vent tubes to a
> "T" fitting, which then vents to atmosphere. What would happen if I removed
> the "T" fitting and just connected one vent tube to the other (i.e. no
> venting to atmosphere)? Seems like any overflow would just flow from one
> bowl to the other. How are yours connected? Do you have a webpage I could
> look at or any photos you can e-mail?
>
> Thanks a lot!
>
> George P.
>
>
> >From: Richard Gosling <richard.gosling@exprogroup.com>
> >Date: Thu, 30 May 2002 12:55:20 +0100
> >
> >George,
> >
[snip]
> >The float chambers do have vents on top, although I've yet to see anything
> >come out of them, they should just be there to balance the air pressure.
> >
> >Good luck with your problem!
> >
> >Richard
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